We spoke to a top venture capitalist who thinks a lot of fast-growing startups like Instagram, Quora, and Klout are overhyped.

He didn't want to be quoted by name, but he's been in the industry for about a decade, and has had several large exits -- including at least one over $1 billion.

"Just because a company has a lot of growth doesn't mean it will be a good business," he told us.

Instagram, for example, had 1 million downloads in 24 hours after it launched its Android version. Its iOS version has over 30 million users. But that doesn't mean it's a sustainable business that can make money for investors over the long haul.

Another example our source pointed to is Quora, which has had a lot of hype over its usage, but hasn't yet had a lot of monetization opportunity. He said that Klout has been hyped up as well.

Some of these companies might still be good investments as they're bought by other companies.

"Many momentum investments end up performing on making money for investors, even if they aren't great businesses," our source said.

One fast-growing company he thinks is NOT overhyped: Pinterest. It will likely have a business from referral fees. They are already referring a lot of traffic, just not monetizing it yet.